Page 1021 - Fundamentals of anatomy physiology

1008 Unit 5 Environmental Exchange The Proximal Convoluted Tubule
Clinical Note
Glomerulonephritis Glomerulonephritis (glo-mer-ū-lō-
nef-RĪ-tis) is an inflammation of the glomeruli that affects
filtration in the kidneys. The condition is often an immune
complex disorder. It may develop after an infection involv-
ing Streptococcus bacteria. p. 848 The kidneys are
not the sites of infection, but as the immune system
responds, the number of circulating antigen–antibody
complexes skyrockets. These complexes are small enough
to pass through the dense layer, but too large to fit
through the filtration slits of the filtration membrane. The
complexes clog up the filtration mechanism. As a result,
filtrate production decreases. Any condition that leads to
a massive immune response, including viral infections and
autoimmune disorders, can cause glomerulonephritis.
26 of the glomerular capillaries. These unusual cells are called
podocytes (PO. -do. -sı.ts; podos, foot + -cyte, cell). Their feet
are known as pedicels (Figure 26–8b). Materials passing out
of the blood at the glomerulus must be small enough to pass
through the narrow gaps, called filtration slits, between adja-
cent pedicels.
Mesangial cells are special supporting cells that lie be-
tween adjacent capillaries. Actin-like filaments in these cells
enable them to contract. In this way these cells control cap-
illary diameter and the rate of capillary blood flow. Several
substances, including angiotensin II, ADH (vasopressin), and
histamine, affect mesangial cell contraction. p. 666 Some
evidence suggests that these cells also make renin.
The glomerular capillaries are fenestrated capillaries. That is, tively removes sodium and chloride ions from the tubular fluid.
their endothelium contains large-diameter pores (Figure 26–8b).
The dense layer differs from that found in the basement mem-
brane of other capillary networks in that it may encircle more
than one capillary.
Together, the fenestrated endothelium, the dense layer,
and the filtration slits form the filtration membrane. During
filtration, blood pressure forces water and small solutes across
this membrane and into the capsular space. The larger solutes,
especially plasma proteins, do not pass through.
Filtration by the renal corpuscle is both effective and pas-
sive, but it has one major drawback: In addition to metabolic
wastes and excess ions, useful substances such as glucose, free
fatty acids, amino acids, vitamins, and other solutes also enter
the capsular space. These substances are reabsorbed before the
filtrate leaves the kidneys. Much of this reabsorption takes place
in the proximal convoluted tubules.
Recall that the renal tubule consists of three segments: the proxi-
mal convoluted tubule, the nephron loop, and the distal con-
voluted tubule. The proximal convoluted tubule (PCT) is the
first segment (Figure 26–6). Its entrance lies almost directly op-
posite the point where the afferent and efferent arterioles connect
to the glomerulus. The lining of the PCT is a simple cuboidal
epithelium whose apical surfaces have microvilli (Table 26–1).
The tubular cells reabsorb organic nutrients, ions, water, and
plasma proteins (if present) from the tubular fluid and release
them into the peritubular fluid, the interstitial fluid surround-
ing the renal tubule. The reabsorbed substances in the peritubu-
lar fluid eventually reenter the blood. Reabsorption is the primary
function of the PCT, but the epithelial cells can also secrete sub-
stances into the lumen of the renal tubule.
The Nephron Loop (Loop of Henle)
The PCT makes an acute bend that turns the renal tubule to-
ward the renal medulla. This turn leads to the nephron loop,
or loop of Henle (Figures 26–6, 26–7). We divide the nephron
loop into a descending limb and an ascending limb. Fluid in
the descending limb flows toward the renal pelvis. Fluid in the
ascending limb flows toward the renal cortex. Each limb con-
tains a thick segment and a thin segment. The terms thick and
thin refer to the height of the epithelium, not to the diameter
of the lumen: Thick segments have a cuboidal epithelium. Thin
segments are lined with a squamous epithelium (Table 26–1).
The entire descending limb is permeable to water. Its thick
descending segment has functions similar to those of the PCT: It
reabsorbs sodium and chloride ions out of the tubular fluid. The
ascending limb is impermeable to water, but passively and ac-
Its thin ascending segment is permeable to sodium ions, which
diffuse into the surrounding peritubular fluid. The thick ascend-
ing segment actively transports sodium and chloride ions out of
the tubular fluid. The effect of this movement is most noticeable
in the renal medulla, where the long ascending limbs of juxta-
medullary nephrons create unusually high solute concentrations
in the peritubular fluid. The descending limb is freely permeable
to water, but not to solutes, such as sodium and chloride ions.
Water moves out of the descending limb, helping to increase the
solute concentration of the tubular fluid.
The Distal Convoluted Tubule
The thick ascending limb of the nephron loop ends where it
forms a sharp angle near the renal corpuscle. The distal con-
voluted tubule (DCT), the third segment of the renal tubule,
begins there. The initial segment of the DCT passes between the
afferent and efferent arterioles (Figure 26–8a).